Mica sheet and tape

ABSTRACT

The present invention relates to a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet structure for electricalinsulation or flame resistance comprising a barrier ply having amica-rich face and a mica-poor face and a reinforcing ply containing asaturable backing layer attached to the mica-poor face of the barrierply.

2. Description of the Related Art

The use of a mica-based sheets and tapes as an electrical insulating andflame barrier material is well known in the art. Such sheets and tapestypically consist of mica paper glued to a backing of reinforcingmaterial.

U.S. Pat. Nos. 4,286,010, 4,769,276, 6,103,382, 6,153,301, and EuropeanPatent Application 0 373 137 A2, disclose the preparation and use ofmica sheets and tapes in electrical insulation and flame protectionapplications. However, mica tapes produced by gluing mica paper to abacking reinforcement have limitations because the glue must becompatible with saturating resins that are commonly applied to the tapesin a subsequent step. The glue must also be thermally stable forextended periods of time. For example, a widely-used combination ofbacking glass cloth and epoxy resin as a glue limits the end-usetemperature of the insulating tape to 180° C., as well as to thepossible resins that may be used in the impregnation step (one normallymust use the same epoxy resin). Also, the process of gluing of manyresins usually involves continuous solvent-based processes and all thetechnological and safety complications related to dealing with volatileorganics (ventilation, recuperation of the solvent and etc.).

An aramid-mica blended paper, made from a homogeneous blend of m-aramidfiber, m-aramid fibrids, and mica is manufactured by DuPont under tradename NOMEX® paper type 418. Such papers are described, for example, inU.S. Pat. No. 6,312,561. Because of superior mechanical properties vs.mica paper, tapes from NOMEX® 418 paper can be used without any backingreinforcement. However, 30-60 weight percent of organic (aramid)components in this barrier material limit its possible corona resistanceor voltage endurance. Another limitation is that during vacuum-pressureimpregnation of an item insulated with a tape based on this paper, it isnecessary to rotate this item until the resin is completely curedbecause the tape is not readily saturable, that is the tape has nocomponent that can hold uncured resin.

A two-ply paper has also been disclosed in which one ply consists ofmica and another ply can consists of aramid, glass, polyester or otherfibrous components (Russian Pat. 2051434, 1996). These papers do nothave the necessary reinforcement and are relatively weak in sheet formand do not make acceptable narrow-width tapes.

What is needed is a sheet structure containing a high concentration ofmica that can be made into tapes and easily saturated with a resin andthat has sufficient mechanical properties for use in electricalinsulation and flame retardant structures.

SUMMARY OF THE INVENTION

This invention relates to a sheet structure comprising a barrier plycontaining a mica-rich face and a mica-poor face, and a reinforcing plyattached to the mica-poor face of the barrier ply, with the reinforcingply comprising a saturable backing layer. The mica content in themica-rich face of the barrier ply is at least 60 weight percent based onthe total components in the mica-rich face; the mica-poor face containsless than 40 weight percent mica, based on the total components in themica-poor face; and the saturable backing layer of the reinforcing plyis a spunlaced aramid material. In the preferred embodiment of thisinvention, the mica-rich face of the barrier ply has greater than 85weight percent mica by weight based on the total components in themica-rich face and the mica-poor face has less than 10 weight percentmica based on the total components in the mica-poor face layer. Thesheet structure of this invention may also include aramid material inthe barrier ply in addition to mica. The amount of mica in the mica-richand mica-poor faces can be measured in the sheet structure by carefullymicrotoming or microcutting the various integrated layers from the sheetstructure and performing standard analytical techniques to analyze thecomposition of each layer.

In addition, the sheet structure of this invention can be used to makemica-based tapes and can be combined with matrix resin to make prepregs.

This invention further relates to a process for making a sheet structurecomprising the steps of forming of a barrier ply containing a mica-richface and a mica-poor face and attaching a reinforcing ply containing asaturable backing layer to the mica-poor face of the barrier ply. Thebarrier ply may be formed on a paper machine, and the mica-rich andmica-poor faces of the barrier ply may be formed by supplying separatemica-rich and mica-poor slurries to the paper machine. Further, thereinforcing ply may be attached to the barrier ply by laminating orcalendaring the plies.

DETAILED DESCRIPTION

The present invention provides a mica-based sheet structure, tape, andprepreg that is useful, for example, as an electrical insulation.Further, the present invention may be used as a flame barrier due to asignificant content of mica in the composition.

A sheet structure or tape of the present invention comprises at least 30weight percent of mica on a total weight basis and is comprised of atleast one barrier ply and one reinforcing ply. The mica is concentratedin one face or side of the barrier ply with the other face or side ofthe barrier ply having no or a low concentration of mica. By mica-richit is meant that the face has a mica content of at least 60 percent byweight, based on the total weight of the components in the mica-richface. By mica-poor it is meant the face has a mica content of less than40 percent by weight, based on the total weight of the components in themica-rich face.

While not intended to be limiting as to the method of manufacture, thebarrier ply of the present invention may be made on a conventionalpaper-making machine equipped with two headboxes for forming a layeredpaper. Two different slurries having the desired proportions of micaand/or other material solids are provided to the two headboxes and thetwo slurries are then wet-laid as layers onto a papermaking wire with,for example, the mica-poor face being formed on the papermaking wire andthe mica-rich face being formed on top of the mica-poor face. Thiscreates a single sheet having an intermingling of the fibrous componentsat the interface of the mica-rich and mica-poor faces.

Once formed, the barrier ply may then be combined with a separatereinforcing ply, with the reinforcing ply facing the mica-poor face ofthe barrier ply. The two plies are then attached to each other andpreferred methods are to attach the two plies together without adhesiveresins or glues by laminating the two plies in the wet press of thepapermaking machine or in the nip of a hot roll calender.

The sheet structure or tape of the present invention can have a basisweight from about 30 g/m² to about 300 g/m² and a thickness from about0.02 mm to about 1.0 mm.

The mica-rich face of the barrier ply in mica-based sheet or tape of thecurrent invention contains at least 60 weight percent, and preferably atleast 85 weight percent of mica, based on total weight of components inthe mica-rich face. Other components of the mica-rich face may includemeta-aramid fibrids and other binder materials; and aramid, glass andother fibers, which can be used for the reduction of dusting from micatape and adjustment of the mechanical properties of the mica-rich ply.

The mica-poor face of the barrier ply can have the same components asthe mica-rich face and preferably contains meta-aramid fibrids as abinder along with aramid fiber or other thermostable floc or fiber ormixtures of such flocs or fibers. By thermostable it is meant that thegiven floc or fiber can withstand a long exposure to the end-usetemperature without significant degradation (usually, to remain at least50% of its initial properties after exposure for 100,000 hours to thenecessary temperature. Other thermostable floc or fibers can include,but are not limited to, such fibers as polyesters andpolybenzimdiazoles, and fibers and flocs based on fluoropolymers, glass,and silica.

The reinforcing ply of the sheet structure of this invention contains asaturable backing layer which may be a woven, knitted or nonwoven fabricprepared from any thermally stable organic or inorganic fibers or theirmixtures of those fibers. Possible fibers include, but not limited to,aramid, polyester, and glass fibers. It is preferred that thereinforcing ply be a spunlaced nonwoven fabric due to the goodcushioning properties such fabrics exhibit during winding of the sheetstructure, along with the fast and uniform resin pick-up that thesenonwoven exhibit during resin impregnation. The reinforcing ply mayinclude other plies or layers to impart other functionality as long asthe saturability of the saturable backing layer is not compromised.

As stated previously, the amount of mica in the faces of the barrier plycan be different depending on the particular end-use with the mica-richface having at least 60 weight percent mica based on the total weight ofcomponents in that face, and the mica-poor face portion having less than40 weight percent of mica based on the total weight of components inthat face. Prior to impregnation by a resin, the barrier ply may bebetween 50 and 95 weight percent based on the total weight of the sheetstructure, and the reinforcing ply may be between 5 and 50 weightpercent, based on the total weight of sheet structure.

Mica of various types (muscovite or phlogopite, thermochemical orthermomechanical) can be used in the present invention. The optimum typefor any particular sheet structure generally depends on the end-useapplication. Typically, muscovite is preferred for electrical insulationand phlogopite is preferred for flame barriers.

The term “floc”, as used herein, means fibers that are cut to a shortlength and which are customarily used in the preparation of wet-laidsheets. Typically, floc has a length of from about 3 to about 20millimeters. A preferred length is from about 3 to about 7 millimeters.Aramid floc is a preferred floc used in the mica-rich and mica-poorfaces of the barrier ply. Floc is normally produced by cuttingcontinuous fibers into the required lengths using well-known methods inthe art.

The term “aramid”, as used herein, means aromatic polyamide, wherein atleast 85% of the amide (—CONH—) linkages are attached directly to twoaromatic rings. Optionally, additives can be used with the aramid andmay be dispersed throughout the polymer structure. It has been foundthat up to as much as about 10 percent by weight of other polymericmaterial can be blended with the aramid. It has also been found thatcopolymers can be used having as much as about 10 percent of otherdiamines substituted for the diamine of the aramid or as much as about10 percent of other diacid chlorides substituted for the diacid chlorideof the aramid.

The term “fibrids”, as used herein, means very small, nongranular,fibrous or film-like particles with at least one of their threedimensions being of minor magnitude relative to the largest dimension.These particles are prepared by precipitation of a solution of polymericmaterial using a non-solvent under high shear. The term “aramidfibrids”, as used herein, means non-granular film-like particles ofaromatic polyamide having a melting point or decomposition point above320° C. The fibrids generally have an largest dimension length in therange of about 0.2 mm to about 1 mm with a length-to-width aspect ratioof about 5:1 to about 10:1. The thickness dimension is on the order of afraction of a micron, for example, about 0.1 microns to about 1.0micron. While not required, it is preferred to incorporate aramidfibrids into barrier plies while the fibrids are still in a never-driedstate. In addition to aromatic polyamide, aramid fibrids can optionallycomprise dyes, pigments, and other additives such as antistatic agents,surfactants, or fillers such as carbon black, silica and titaniumdioxide.

Either ply of the sheet structure and/or tape of this invention maycontain special additives for further improvement of voltage endurancebehavior. Such additives can include but are not limited to particlesbased on silicone dioxide, aluminum oxide, titanium dioxide, zirconiumdioxide or their combinations.

The sheet structure and/or tape of this invention, as made, preferablydoes not contain any adhering resin between the plies, which allows theuse of a broad range of resins during impregnation of the sheet or tapeto make the final insulation material.

Several methods may be utilized for the preparation of a finalresin-impregnated insulation material based on the sheet structureand/or tape of this invention. The first common method comprisesimpregnating the tape or sheet with resin after it has been insertedinto, or wound around, the item to be insulated and then curing theresin. The second common method comprises impregnating the tape or sheetwith resin prior to it being inserted into or wound around the item tobe insulated and then curing the resin.

It is surprising and unexpected that in the preferred version of thisinvention the mica-poor face of the barrier ply can provide good bondingbetween the barrier ply and the reinforcing ply without using anyadditional adhering resin. It is known in the art that one ply of aramidpaper or board can be laminated in a hot calender nip or in the hotpress with another ply of aramid paper or board, if both of the pliescontain fibrids, moldable fibers or some other bonding agents, as itdescribed, for example, in U.S. Pat. Nos. 4,481,060 and 4,752,355.However, it is unexpected that sufficient bonding and sheet strength canbe achieved by lamination of the barrier ply with a reinforcing plycontaining a saturable backing material which doesn't contain anyfibrids, moldable fibers, or glue; or that the saturable surface willhave adequate porosity for further resin-impregnation after suchlamination.

As used herein, “prepreg” includes a mica-based sheet or tapeimpregnated by a resin and having enough formability to be compressedand/or shaped into a final composite. When the sheet or tape isimpregnated with a thermoset resin, the resin is usually in a curingstage B (partially soluble) in the prepreg and can be cured additionallyto stage C (cross-linked, not soluble) later.

A prepreg based on the mica-based sheet or tape of the present inventioncan be made by any known technique by the use of a matrix resinsolution, dispersion, or melt which will flow into the structure of thepresent invention. The viscosity of the matrix resin is not especiallycritical to this invention. If the matrix resin has a very highviscosity it should be dissolved in a solvent to form a solution andthen the solution can be used to impregnate the sheet. If a lowviscosity matrix resin is utilized, the solution or dispersion is notnecessary. For example, low-viscosity epoxy compositions developed forvacuum-pressure impregnation (VPI) are in use in the industry now andcan be used with the materials of present invention without any additionof solvents.

Test Methods

The following test methods were used in the Examples provided below. Thetensile properties of mica-based sheet and tape of the present inventionwere measured on an Instron-type testing machine using test specimens2.54 cm wide and a gage length of 18 cm, in accordance with ASTM D828-93.

The thickness and basis weight of mica-based sheet of present inventionwere determined by measuring the thickness and the weight of an area ofa sample of the test nonwoven sheet in accordance with ASTM D 645/D645-M-96 and ASTM D 646-96, respectively.

Internal tearing resistance of mica-based sheets was measured in asingle sheet in accordance with ASTM D689-96a with the usage ofElmendorf-Type Tearing Tester.

Resin pick-up was determined based on measuring the weight of amica-based sheet before impregnation and after impregnation and completecuring of a resin. Before each weighing, the material was dried in theoven at 110 C. for 24 hours. Resin pick-up ir weight percent wascalculated based on the equation:{(Wt. After−Wt. Before)/(Wt. After)}×100%,Where Wt. Before and Wt. After is weight of the material before andafter impregnation, respectively.

EXAMPLE

An aqueous dispersion containing a mixture of meta-aramid floc andmeta-aramid fibrids was made having a relative content of non-aqueouscomponents (percents by weight) as follows:

-   -   Meta-aramid floc about 60 weight percent,    -   Meta-aramid fibrids about 40 weight percent.        Another aqueous dispersion containing a mixture of mica and        meta-aramid fibrids was made having a relative content of        non-aqueous components (percents by weight) as follows:    -   Mica about 95 weight percent,    -   Meta-aramid fibrids about 5 weight percent.

The dispersions were pumped through primary and secondary headboxes of aFourdrinier-type papermaking machine and a layered wet-laid ply wasformed with a mica-rich layer on the top and a mica-poor layercontaining predominantly aramid components on the bottom. The basisweight of the top layer was about 84.8 g/m2 and basis weight of thebottom layer was about 50.9 g/m2.

The mica used was muscovite type, Electrical Samica Flake (sold by USSAMICA Incorporated, Rutland, Vt., USA).

The meta-aramid fibrids were made from poly(metaphenyleneisophthalamide) in a manner generally described in U.S. Pat. No.3,756,908.

The meta-aramid floc was poly(metaphenylene isophthalamide) floc oflinear density 0.22 tex and length of 0.64 cm (sold by E. I. du Pont deNemours and Company under the trade name NOMEX®).

The layered wet-laid ply formed the barrier ply and was laminated with areinforcing ply of nonwoven meta-aramid material in the hot nip of acalender at nip pressure of about 3000 N/cm. The temperature of thebottom role (faced to spunlaced material) was about 350° C., and thetemperature of the top roll (faced to the mica-rich ply) was about 180°C. The nonwoven meta-aramid material used as the reinforcing ply waspoly(metaphenylene isophthalamide) spunlaced fabric having a basisweight of 68 g/m2 (sold by E. I. du Pont de Nemours and Company asNOMEX® spunlaced fabric type E88, style 320).

The resulting mica-based sheet structure had a basis weight about 205g/m2 and a thickness about 0.208 mm. About 66.1 weight percent of thetotal weight of the sheet was contained in the barrier ply with a micacontent of 95 weight percent in the mica-rich face based on mica-richface components, and essentially no mica in the mica-poor face. About33.9 weight percent of the total weight of the sheet was contained inthe reinforcing layer. The total mica content of the sheet on a totalweigh basis was about 39.2 weight percent. Other properties of the finalmica-based sheet are shown in Table 1. A part of the prepared mica-basedsheet was then slit into mica-based tapes with width about 12.7 mm.Another part of the mica-based sheet was cut into 20 cm×20 cm pieces andimpregnated with a solvent-free low-viscosity epoxy composition typeE-833 (sold by P.D. George Co., St. Louis, Mo., USA). To simulate avacuum-pressure impregnation process, a piece of the sheet was dried inthe oven at 110° C. for 24 hours, then impregnated with the resin in avacuum oven at room temperature with residue pressure of about 5 kPaand, finally, heat treated in a platen press in a multi-step process atconstant pressure of about 650 kPa and temperature about 135 C. for 60min., 156° C. for 120 min., 177° C. for 60 min., and 93° C. for 1 min.

Comparison Example

The barrier ply of the previous example was calendered without thereinforcing material at the same conditions as were used in thatexample. The calendered paper had thickness of about 0.105 mm. Someother properties are shown in Table 1.

As it can be seen from Table 1, the reinforcing ply of Example 1provided a significant increase in tensile and tear properties of themica-based barrier sheet, as well as an increase in resin pick-up. Thisconfirmed that bonding through the mica-poor layer of the barrier plywas adequate, and that the saturable surface of the reinforcing ply wasstill saturable by resins after the lamination process. TABLE 1 TensileInitial Tear Initial Tear Resin Strength Resistance Resistance Pick-Upin Machine in Machine in Cross During Direction, Direction, Direction,Impregnation, Material N/cm N N weight percent Example 41.3 13.2 14.8 47Comparison 27.1 2.0 3.0 36 Example

1-8. (canceled)
 9. A process for making a sheet structure comprising thesteps of: a.) forming of a barrier ply containing a mica-rich face and amica-poor face; and b.) attaching a reinforcing ply to the mica-poorface of the barrier ply, said reinforcing ply comprising a saturablebacking layer.
 10. The process of claim 9 wherein the barrier ply isformed on a paper machine.
 11. The process of claim 10 wherein themica-rich and mica-poor faces of the barrier ply are formed by supplyingseparate mica-rich and mica-poor slurries to the paper machine.
 12. Theprocess of claim 9 wherein the saturable backing layer of thereinforcing ply is a spunlaced material.
 13. The process of claim 9wherein the reinforcing ply is attached to the barrier ply by laminatingor calendaring the plies.
 14. (canceled)